1. Field of the Invention
This invention relates generally to the field of memory devices and, more particularly, to a memory module with integrated bus termination.
2. Description of the Related Art
Typical computer systems generally include a processing unit, such as a microprocessor, dynamic memory resources, data storage resources, and other peripheral resources, such as I/O resources. A circuit board is provided for interconnecting these various resources. Some of the resources are installed directly on the circuit board, while other resources connect to the circuit board through expansion sockets. For instance, a particular circuit board may have surface-mounted memory modules, memory modules that interface through one or more expansion sockets, or a combination of both.
Commonly used memory modules that interface through expansion sockets through edge connectors are multi-chip modules (MCMs). Generally, MCMs may be designed to include more than one type of die within a single package, or may include multiples of the same die, such as a single inline memory module (SIMM) or a dual in-line memory module (DIMM). MCMs are available in different capacities and speeds to accommodate the needs of the computer system.
The memory resources, whether installed directly on the circuit board or through expansion sockets, are coupled to a common memory bus, where data, address, and command signals are provided in parallel to the various modules. The memory bus thus includes a plurality of transmission lines. The operating environment (e.g., speed, bus load) and physical characteristics (e.g., length) of the memory bus affect the characteristics of the transmission lines. Signals traveling along the transmission lines may be reflected at the ends of the bus, thus interfering with the signals. In general, as the speed of the memory bus increases or the load on the memory bus increases the sensitivity of the bus to the reflections also increases.
One technique for reducing the effects of reflections on the memory bus is to terminate the transmission lines of the bus. Although various termination schemes may be employed, a common termination scheme is to use a pull-up resistor coupled to a voltage source, Vtt, that is typically half the voltage corresponding to a logic one on the bus. For example, if a logic one is represented by a voltage of 1.8 V, the termination voltage, Vtt, may be 0.9 V. One limitation of the bus termination technique is the relatively large surface area of the circuit board that is consumed by the termination circuitry. The termination circuitry increases the cost of the circuit board and also reduces the area available for other resources.
Depending on the particular resources installed on the circuit board, the termination may or may not be necessary. For example, if only on-board memory is used, the load on the bus may be sufficiently small that the reflections do not significantly affect the operation of the bus. In such an installation, the termination circuitry represents an unnecessary expenditure of resources. In other installations, where one or more memory modules are provided in expansion sockets, the termination circuitry may be required.
Typically, the central circuit boards, otherwise referred to as motherboards, and the memory modules are provided by different manufacturers. Dependence on termination circuitry provided by the circuit board manufacturer limits the flexibility of the memory module manufacturer, in that its memory modules may only be used in selected systems.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.